Known optical signal modulation devices comprise first and second optical waveguides connected at one end to an optical signal splitting device which couples portions of an incoming optical signal into the waveguides and at the other end to an optical signal combining device which causes optical signals received from the waveguides to interfere; and first modulating means responsive to a first control signal to cause a relative phase shift between signals in the first and second optical waveguides, the first modulating means causing relative phase shifts in both TM mode and TE mode signals. Such devices are hereinafter referred to as "of the kind described". Examples of known devices of the kind described are lithium niobate integrated optical Mach-Zehnder modulators.
One major problem with known modulation devices of the kind described is that they display marked polarisation sensitivity. In any practical telecommunications system this can act as a severe limitation since the state of polarisation of the signal emerging from an optical waveguide (such as a length of optical fibre) can vary in a random fashion. Previous proposals for developing a lithium niobate polarisation insensitive modulator involve the use of off axis propagation. These approaches are limited by significantly increased insertion loss and difficulty of fabrication respectively.
The effect of polarisation sensitivity is that the transfer characteristic of the device for TM and TE modes is significantly different. This leaves a particular problem when the device is used as an optical switch in which a two state control function such as a voltage can be determined which correctly switches one mode but which has little useful effect on the other mode. This is a particular problem when it is desired to turn off the switch. The non-coincidence of minima of the two transfer characteristics means that a proportion of one zero mode polarisation will continue to pass through the device when it is controlled in the off state.
In accordance with the present invention, an optical signal modulation device of the kind described is characterised in that the device further comprises second modulating means responsive to a second control signal to cause a relative phase shift between signals of one mode in the first and second waveguides and to leave signals of the other mode substantially unaffected whereby the transfer characteristics of the two modes may be shifted relatively to one another in use in accordance with the second control function.
With this invention, a second modulating means is provided to cause a second relative phase shift between signals of one mode in the first and second waveguides. By a suitable choice of second control function, it can be arranged that a minimum of the transfer characteristic of the one mode is substantially coincident with a minimum of the transfer characteristic of the other mode.
This property finds particular application for devices fabricated from lithium niobate, particularly Z cut lithium niobate since the electro-optic coefficients of lithium niobate which interact with the first modulating means have a nearly integral relationship (in fact related by a factor of 3). Thus, when the device is to be used as a switch, the two state control function applied to the first modulating means can be arranged to switch between potentials related by the same factor of 3 since this will have substantially the same effect on both the TM and TE optical modes.
Although in this application the maxima will not exactly coincide, this is acceptable particularly for digital signals.
Although in theory, for certain devices, it may be possible for the second modulating means to act on the TM mode independently of the TE mode, preferably the second modulating means causes a relative phase shift between TE mode signals in the two waveguides.
Typically, the first and second modulating means each comprise electric field generating means for generating respective electric fields which are substantially orthogonal to each other and to the waveguides. In this case, the electric field generated by the second modulating means extends across only one of the waveguides. Typically, the second modulating means will apply a substantially constant bias field.
In this specification, the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
Preferably, when the device is incorporated into an optical switch, the output of the combining device is coupled with a single mode waveguide. This provides a simple way of causing the device to act as a switch since when the first control function takes up one state, only first or higher order mode signals will be generated at the combining device and will therefore not be guided by the single mode waveguide. When the first control function takes up its other state, zero order modes will result which are guided by the single mode waveguide.